Epoxy Tips – Exact Mixing Ratios

Pumps make measuring the correct ratios of resin to hardener very easy. Make sure you have the right pump for the container – these are 301A for the A pack size (1kg of WEST SYSTEM 105 epoxy resin with 0.2kg of 205 Fast or 206 Slow hardener) and 301B for the B pack size (5kg of 105 epoxy resin with 1kg of 205 Fast or 206 Slow hardener). There are also 303 pump sets for use when working with 207 Special Coating or 209 Extra Slow hardeners.

Epoxy is very easy to use but has to be mixed in exactly the right ratio of resin to hardener. This differs from polyester resin, where the curing process has already been ‘accelerated’ and just requires a small percentage of catalyst (usually around 2%, although professionals will tweak it) to bring on the final set.

Fortunately, if your maths isn’t quite up to working out resin/hardener ratios such as 5:1 and 3:1 by volume, the calculations are done automatically by the use of specially calibrated WEST SYSTEM Mini Pumps.

Essentially, one full stroke of the resin pump and one full stroke of the smaller hardener pump will measure out exactly the right ratio of resin to hardener every time. (Don’t try half strokes unless priming – you’ll lose accuracy)

However, it is very important that the pumps are matched to the resin and hardener type they are dispensing. They are supplied for DIY use in pairs for this very reason, so a 301 resin pump for 105 epoxy resin, for example, is calibrated for a 205 or 206 hardener, which both have a 5:1 ratio.

If you want to use a different hardener, such as the 207 Special Coating hardener (great for priming wood for varnishing with a rich, deep gloss) or 209 Extra Slow hardener (for use in very hot regions) you will need the 303 Mini Pump sets. An alternative is to measure out the ratios in a calibrating cup by volume, as the 207/209 hardener is mixed at 3:1. (Or for every 300ml of resin, you use 100ml of hardener.)